Extensive prior art exists in the application of two terminal negative resistance devices developed after the publication by Leo Esaki of "New Phenomenon in Narrow Germanium p-n Junctions", Phys. Rev., Vol. 109, p. 603, Jan. 15, 1958. However, as the title of the article indicates, a resonant tunnel diode (RTD) with its plurality of thin semiconducting layers having different physical properties is markedly different from Esaki's single p-n junction. Although the shape of the plot of device current vs. voltage is superficially similar, the scale is grossly different. In addition, the manufacturing technology available to achieve massive arrays of virtually identical RTDs is not available for Esaki tunnel diodes. In its simplest form, an RTD consists of a sequence of five semiconductor layers. The outer two layers are the contact layers into which electrons enter and exit the semiconductor layer sequence. The interior three dissimilar semiconductor layers differ in their energy band gaps in the sequence of wide/narrow/wide band gap with layer thicknesses comparable to the electron Bloch wavelength (typically less than 10 nm). The electron path through these layers consists of two energy barriers separated by a narrow region referred to as a quantum well.
Classically, an electron with energy, called Fermi energy, approaching the first energy barrier with an energy below the barrier energy is reflected. As the physical dimensions of the barrier decrease toward the wavelength of the particle, there is an increasing probability that the particle will be transmitted instead of reflected. Thus, under certain conditions, an electron can pass through the barrier with energy below the barrier potential. This phenomenon is called tunneling.
If the quantum well width is selected to be approximately equal to some half integer multiple of the electron wavelength, a standing wave can be built up by constructive interference analogous to the standing waves in a transmission line or microwave cavity. Electrons at these wavelengths couple into and out of the quantum well more readily than others.
Since the electron's energy can be controlled by adjusting the voltage across the structure, the current flow through the double barrier is quite sensitive to this applied voltage. For certain applied voltages, the electrons readily pass through the double barrier and for other applied voltages are almost totally reflected. The electron is said to be in resonance when the incoming electron energy matches the resonant transmission energy of the quantum-well structure.
The prior art shown in FIG. 1, "Resonant Tunneling Transistors," TI Technical Journal, p.30, Jul-Aug 1989, shows the current vs. voltage (I-V) characteristic and the corresponding conduction band profile of an RTD. The interesting feature of this characteristic is that between points B and C where an increase in applied voltage actually causes a decrease in device current. This region of "negative differential resistance" is highly unstable. The ratio of the voltage at B to C is referred to as the peak-to-valley ratio and is quite important in determining the noise margins of RTDs applied to digital devices. The previously mentioned scale differences in these characteristics between RTDs and older Esaki single junction tunnel diodes is quite dramatic. RTDs can be fabricated whose peak currents are reproducible from device to device and may range from 100 picoamps to greater than 100 milliamps while maintaining useable peak to valley ratios. Esaki diodes operate in the tens of milliamps regime. This allows RTDs to be tailored to the application. In memory devices where power and size are the prime concerns, very low current devices are the choice. In high speed logic where capacitive loads may occur, high current devices are desired.
As has been done in prior art, adding a resistor in series with the diode as illustrated in FIG. 2 achieves a bistable circuit whose static resting point is either 10 or 12. By applying a control voltage at 14, the device can be forced to either 10 or 12 making it into a resettable binary latch. This structure is disadvantageous because of (1) power dissipation, (2) manufacturability and (3) operating speed among other problems.
A more suitable structure is to series connect two RTDs in a manner similar to that proposed by Goto, et al in IRE Trans. on Electronic Computers, March 1960, p. 24 using conventional Esaki tunnel diodes and studied extensively by RCA, RCA Review, vol XXXIII, June 1962, p. 152 and Dec. 1962, p. 489. The series connection of two RTDs and their bistable operating points 16 and 18 is illustrated in FIG. 3. It is this configuration which is exploited by this invention.
Modern digital systems require the acquisition, transfer and storage of digital bit streams at gigahertz clock frequencies. Most systems in this frequency range use a continuous or synchronous clock with dynamic shift registers. A dynamic shift register stores the binary ones and zeroes in capacitors, normally the parasitic capacitances of the circuit interconnects. However, in many systems the data stream is asynchronous, i.e. stops and starts. However, in a dynamic system, the data may be lost due to capacitor leakage if the clock is stopped for too long a period. The faster the clock the smaller the capacitor and the sooner the data is lost if the clock stops. A static shift register can be used to overcome this difficulty. However, static circuits require more components and hence device and interconnect delays. Many times these circuits also dissipate considerably more power than their dynamic counterparts. As seen in FIG. 3, both stable points 16 and 18 are at low current.